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Conversion of units
Conversion of units is the conversion between different units of measurement for the same quantity, typically through multiplicative conversion factors. Techniques Process The process of conversion depends on the specific situation and the intended purpose. This may be governed by regulation, contract, Technical specifications or other published standards. Engineering judgment may include such factors as: * The precision and accuracy of measurement and the associated uncertainty of measurement * The statistical confidence interval or tolerance interval of the initial measurement * The number of significant figures of the measurement * The intended use of the measurement including the engineering tolerances Some conversions from one system of units to another need to be exact, without increasing or decreasing the precision of the first measurement. This is sometimes called soft conversion. It does not involve changing the physical configuration of the item being measured. By contrast, a hard conversion or an adaptive conversion may not be exactly equivalent. It changes the measurement to convenient and workable numbers and units in the new system. It sometimes involves a slightly different configuration, or size substitution, of the item. Nominal values are sometimes allowed and used. Multiplication factors Conversion between units in the metric system can be discerned by their prefixes; for example, 1 kilogram = 1000 grams, 1 milligram = 0.001 grams) and are thus not listed in this article. Exceptions are made if the unit is commonly known by another name (for example, 1 micron = 10−6 meter). Table ordering Within each table, the units are listed alphabetically, and the SI units (base or derived) are highlighted. Tables of conversion factors This article gives lists of conversion factors for each of a number of physical quantities, which are listed in the index. For each physical quantity, a number of different units (some only of historical interest) are shown and expressed in terms of the corresponding SI unit. Length Area Volume Plane angle Solid angle Mass Notes: * See Weight for detail of mass/weight distinction and conversion. * In this table, the unit gee is used to denote standard gravity in order to avoid confusion with the "g" symbol for grams. * In , the pound of mass is sometimes written lbm to distinguish it from the pound-force (lbf). It should not be read as the mongrel unit "pound meter". Density Time Frequency Speed or velocity A velocity consists of a speed combined with a direction; the speed part of the velocity takes units of speed. Flow (volume) Acceleration Force See also: Conversion between weight (force) and mass Pressure or mechanical stress Torque or moment of force Energy Power or heat flow rate Action Dynamic viscosity Kinematic viscosity Electric current Electric charge Electric dipole Electromotive force, electric potential difference Electrical resistance Capacitance Magnetic flux Magnetic flux density Inductance Temperature Information entropy Often, information entropy is measured in shannons, whereas the (discrete) storage space of digital devices is measured in bits. Thus, uncompressed redundant data occupy more than one bit of storage per shannon of information entropy. The multiples of a bit listed above are usually used with this meaning. Other times the bit is used as a measure of information entropy and is thus a synonym of shannon. Luminous intensity The candela is the preferred nomenclature for the SI unit. Luminance Luminous flux Illuminance Radiation - source activity Please note that although becquerel (Bq) and hertz (Hz) both ultimately refer to the same SI base unit (s−1), Hz is used only for periodic phenomena, and Bq is only used for stochastic processes associated with radioactivity. Radiation - exposure The roentgen is not an SI unit and the strongly discourages its continued use. Radiation - absorbed dose Radiation - equivalent dose Although the definitions for sievert (Sv) and gray (Gy) would seem to indicate that they measure the same quantities, this is not the case. The effect of receiving a certain dose of radiation (given as Gy) is variable and depends on many factors, thus a new unit was needed to denote the biological effectiveness of that dose on the body; this is known as the equivalent dose and is shown in Sv. The general relationship between absorbed dose and equivalent dose can be represented as :H = Q · D where H'' is the equivalent dose, ''D is the absorbed dose, and Q'' is a dimensionless quality factor. Thus, for any quantity of ''D measured in Gy, the numerical value for H'' measured in Sv may be different. Software tools Home and office computers come with converters in bundled spreadsheet applications or can access free converters via the Internet. Units and measurements can be easily converted using these tools, but only if the units are explicitly defined and the conversion is compatible (e.g., cmHg to kPa). General commercial sources of converters *Advanced electronic calculators have unit-conversion functionality. *Spreadsheet programs usually provide conversion functions or formulas or the user can write their own. *Commercial mathematical, scientific and technical applications often include converters. See also *Accuracy and precision *Conversion of units of temperature *English units *False precision *Imperial units *International System of Units *Mesures usuelles *Metric system *Natural units *Rounding *Significant figures *SI prefix (e.g. "kilo-" prefix) *United States customary units *Units (software) *Units conversion by factor-label *Units of measurement References ;Notes External links *[http://www.unc.edu/~rowlett/units/ ''How Many? A dictionary of units of measurement] * *NIST Guide to SI Units Many conversion factors listed. *The Unified Code for Units of Measure *Units, Symbols, and Conversions XML Dictionary Category:Unit comparison tables